High voltage measuring device

ABSTRACT

High voltage power lines are provided with a coupling capacitor for connecting carrier systems to the power line and for providing a voltage divider to indicate the line voltage. The coupling capacitor usually comprises a series of individual capacitors that provide the necessary voltage rating. A frequency and load independent measuring device comprises first and second input terminals adapted to be connected across one of the capacitors of the series. A measuring capacitor has one end connected to the first input terminal. A load impedance of any finite value is connected between the other end of the capacitor and the second input terminal. Means are provided for measuring the voltages between the second input terminal and each end of the measuring capacitor, and subtracting the differences of selected ratios of the measured voltages to provide an accurate measurement of the voltage of the high voltage line.

United States Patent Br'enig HIGH VOLTAGE MEASURING DEVICE Primary E.\'uminerAlfred E. Smith [75] Inventor; Th d B i Lynchburg Va. Assistant E.\'uminerErnest F. Karlsen [73] Assignee: General Electric Company,

Lynchburg. Va. [5 ABSTRACT [22] Filed: Nov. 26, 1974 High voltage power lines are provided with a coupling capacitor for connecting carrier systems to the power [J] Appl. No.. 527,300 line and for providing a voltage divider to indicate the line voltage. The coupling capacitor usually comprises Related US. Application Data a series of individual capacitors that provide the nec- [63] Continuation-in-part of Ser. No. 458.524. April 8. essury Voltage rating- A frequency and load p 1974. abandoned. dent measuring device comprises first and second input terminals adapted to be connected across one of [52] US. Cl 324/126; 317/12 B; 323/93 the capacitors of the series. A measuring capacitor has [51] Int. Cl. G01R 19/00; HO2l-l 7/16 one end connected to the first input terminal. A load [58] Field of Search 324/126; 317/12 R. 12 B; impedance of any finite value is connected between 323/93 the other end of the capacitor and the second input terminal. Means are provided for measuring the voltages between the second input terminal and each end [56] References C'ted of the measuring capacitor. and subtracting the differ- FOREIGN PATENTS OR APPLICATIONS ences of selected ratios of the measured voltages to 1.083.424 6/1960 Germany 324/126 provide an accurate measurement of the voltage of the 1,055,120 4/1959 Germany 324/126 high voltage line.

4 Claims, 5 Drawing Figures 1 X1 "7" 11 i if 1000! T I3 cm if i H X2= TI T 2 1' I2 (:2 v 2 V3 6 r i l n: %a or1 U.-S. Pat nt' fiov. 11,1975 Sheet2of3 3,919,633

FIG. 3

LOAD

.JTS v 3 FIGA US. Patent Nov. 11, 1975 Sheet 3 of3 FIG. 5

HIGH VOLTAGE MEASURING DEVICE CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my pending application entitled High Voltage Measuring Device filed Apr. 8, I974, Ser. No. 458,524, now abandoncd.

BACKGROUND OF THE INVENTION My invention relates to a high voltage measuring device, and particularly to a high voltage measuring device for providing an accurate measurement of high voltage. alternating current.

Protective devices for 60 Hz, high voltage power lines require an accurate measurement of the power line voltage. The protective devices should be operated when conditions require such operation to protect the line and equipment, but should not be erroneously or falsely operated and thereby interrupt primary electrical power unnecessarily.

Accordingly. a primary object of my invention is to provide a new high. voltage measuring device that is particularly useful on 60 Hz electrical power lines.

Voltage measuring devices for electrical power lines have been previously provided. Typically, such devices comprise a coupling capacitor that has a series of individual capacitors which provide the necessary voltage rating, and that is connected between the power line and ground. A network comprising a tuning inductor and a load (or burden) transformer is connected across one of the individual capacitors, usually the capacitor in the series nearest or connected to ground. Control and indicating circuits forming the load are connected to the secondary of the transformer. While such devices have performed well, they are sensitive to line frequency and to the load.

Accordingly, another object of my invention is to provide a new and improved alternating current, high voltage measuring device that provides a voltage measurement that is completely independent of the frequency of the alternating current and of the load of the measuring circuit.

SUMMARY OF THE INVENTION Briefly, these and other objects are achieved in accordance with my invention'by a'measuring device having first and second input terminals which are adapted to be connected across one or more of the series of capacitors forming a high voltage divider that is to be connected to thehigh voltage line. A" measuring capacitor of selected magnitude has one end connected to the first inputterminal. A loa d or impedance, which may be the primary winding of an output transformer, of any finite magnitude isconnected between the other end of the measuringcapacitor and the second input terminal. Means are connected between the second input terminal and the ends of the measuring capacitor for measuring the relative voltage of the ends of the measuring capacitor with respect'to the second terminal. A predetermined ratio of each of these measured voltages is taken. The ratios of the voltages are subtracted to provide an accurate, frequency and load independent measurement ofthe high voltage on the power line.

BRIEF DESCRIPTION OF THE DRAWING" The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the claims. The structure and operation of my invention, together with further objects and advantages, may be better understood from the following description given in connection with the accompanying drawing, in which:

FIG. 1 shows an electrical circuit diagram of a prior art, high voltage measuring device;

FIG. 2 shows an electrical circuit diagram of a high voltage measuring device in accordance with my invenj FIG. 3 shows an electrical circuit diagram of a high voltage measuring device which is in accordance with my invention, and which is provided with a circuit for deriving and indicating the measured voltage;

FIG. 4 shows an electrical circuit diagram of a high voltage measuring device which is in accordance with my invention, and which is provided with one preferred circuit for deriving and indicating the measured voltage; and

FIG. 5 shows an electrical circuit diagram of a high voltage measuring device which is in accordance with my invention, and which is provided with another preferred circuit for deriving and indicating the measured voltage.

DESCRIPTION OF PREFERRED EMBODIMENTS Before describing my invention, I. will describe a prior art, high voltage measuring device. In FIG. 1', I have shown such a device as used with a high voltage transmission line 10. A coupling capacitor 11, indicated in the dashed line rectangle, is connected to the high voltage line 10 and a point of reference potential, such as ground. Generally, the coupling capacitor 11 comprises a series of generally similar individual capacitors which provide the necessary voltage rating determined by the line 10. The capacitors nearest the line 10 are indicated by the legend Cl, and the capacitor nearest ground is indicated by the legend C2. Since the capacitor C2 is nearest ground, it is the safest and most convenient for measuring the line voltage. If the capacitors in the series are similar, the line voltage is approximately equal to the measured voltage across the capac itor C2 multiplied by the number of capacitors in the series. Terminals from the capacitor C2 are connected to a load transformer 12 which provides the desired isolation. I have shown the transformer 12 as comprising two primary windings l3, 14, each of which has one end connected to a respective side of the capacitor C2. A tuning inductor L1 is used to connect the other ends of the primary windings l3, 14 together and is used to tune out the capacity present in the circuit. A bypass capacitor C3 may also be connected between the lower end of the inductor L1 and ground. The transformer 12 may be provided with one or more secondary windings l5, 16 which in a typical application are used to control relays or other circuit devices which respond to an undervoltage or an overvoltage to cause appropriate switching or interruption of the high voltage line 10.

An examination of the known circuit of FIG. I will show that the voltage provided at the secondary windings 15, 16 is frequency and load dependent. While the frequency on the line 10 may not vary appreciably in some applications, it is desirable that the voltage indication provided in such instances be as stable as possi- 3 ble and as independent of frequency sensitive elements as possible. And clearly the voltages at the secondary windings I5, I6 and at the primary windings 13, I4 depend on-the actual impedance or load connected to the secondary windings l5, I6. Accordingly, I have in- -vented a frequency load independent measuring device as shown in FIG. 2. While other applications are possible. I have shown my measuring device being used in the same application as shown in FIG. 1, namely with the high voltage line I and the coupling capacitor having all but one of the capacitors indicated by the legend CI and the last capacitor indicated by the legend C2. The measuring circuit in accordance with my invention comprises first and second terminals T1, T2 which are adapted to be connected across any one or more of the individual capacitors in the series, but preferably the capacitor C2 nearest ground. A measuring capacitor CM has one end connected to the first terminal TI, and has the other end connected through load to the secondterminal T2. The load 20 may be of any finite magnitude and may be ofany type, namely resistive, inductive, or capacitive. or a combination of these. Generally, such as in FIG. I, the load 20 would be the transformer I2.and the load on the secondary windings 15, I6. I have also indicated certain voltages and currents in FIG. 2, namely a voltage VI between the terminal T2 and the line 10, a voltage V2 between the terminal T2 and one side of the measuring capacitor CM, a voltage V3 between the terminal T2 and the other side of the measuring capacitor, a current ll flowing through the capacitor C], a current I2 flowing through the capacitor C2, and a current I3 flowing through the measuring capacitor CM and the load 20. As will be shown mathematically below, if the voltages V2 and V3 are measured, if predetermined ratios of these voltages are taken, and if the ratios of these voltages are subtracted, the resultant difference is an accurate measurement of the voltage V! between ground or some reference point and the line 10. And further, the measurement is frequency and load independent.

- Inorder to show how this frequency and load independentmeasurement can be obtained, I have assumed that the capacitor CM is an unspecified type of impedance ZM, that the capacitor CI has an impedance X I, and that the capacitor C2 has. an impedance X2. With the circuit of FIG. 2, the following equations can be written: 3

L I2 7 Equation l V2 H E 2 M quation II I2 I3 Equation 3 VI II -XI+V2 Equation4 Substituting equations 1 and 2 for I2 for I3 in equation 3, and substituting that result for llin equation 4, I obtain:

v2 v2-v3 X V7 E If 5 V l X '2 -ZM I qua non Rearranging equation 5, I obtain:

XI XI XI VI V2 I Equation 6 X2 ZM ZM and this can be written as:

. C M V3 7 Equation 8 In equation 8, it will be seen that the line voltage V1 is equal to the voltage V2 multiplied by a constant equal to the sum of the capacities Cl,C2, and CM divided by the capacity Cl, minus the voltage V3 multiplied by the capacity CM and divided by the capacity C1. Since the capacities are constant, the voltage V1 is independent of frequency and is independent of the load 20. However, as pointed out, only a capacitive device CM will provide a frequency independent measurement. lfa resistor or an inductor is substituted for the capacitor CM or is used with the capacitor CM, then the voltage measurement will be frequency dependent. Hence, only the capacitor CM may be used. It should be noted that the measuring capacitor CM may be moved from its location shown in FIG. 2 and placed between the load 20 and the terminal T2. In such a case, the above relations still apply. I

Persons skilled in the art will appreicate that the measured voltage V2 can be multiplied by the ratio and the measured voltage V3 can be multiplied by the ratio suitable electronic circuits such as an operational amplifier having the two inputs connected to receive the voltages V2, V3 in the proper ratios and polarities. In FIG. 3, I show a preferred arrangement for providing an indication of the voltage V]. This arrangement comprises a bridge network having two impedances Z3, Z4 serially connected between one end of the capacitor CM and the terminal T2, and two impedances-Z6, Z5 serially connected between the other end of the capaci'-- f tor CM and the terminal T2. A terminal T4 is provided vat the junctionof the impedances Z3, Z4 and a terminal T5 is provided at the junction of the impedances Z6, Z5. A transformer 30 is connected between the terminals T4, T5 to measure the difference in the voltages V4 and V and. if desired. provide a voltage transformation. With respect to equation 8, the circuit of FIG. 3 should be arranged so that:

VI V4 V5), l/K Equation 9 In these equations, the factor K should be selected so that optimum circuit values can be obtained. Thus, the impedances Z3, Z4 can be selected to provide the voltage V4 in accordance with equation 9, and the impedances Z5, Z6 can be selected to provide the voltage V5 in accordance with equation 9. While resistors could be used for these impedances Z3, Z4, Z6, Z5, in view of the high voltages involved, such resistors would have to have high magnitudes in order to limit power dissipation. Such high magnitudes could cause accuracy problems, so that capacitors are preferable. And, it is also preferable that the impedance 23 be zero and the impcdance Z4 be infinite so that two capacitors can be eliminated. Such a circuit is shown in HQ 4. In this-circuit, the voltage V4 is equal to the voltage V2. ln equation 9,

In equation 9,

If this term is inserted for K in the equation for V5, I

obtain:

V5 V3 Equation It) With respect to FIG. 3, the following voltage and impedance relation exists:

lf equations 10 and ll are combined, I obtain:

Thus, the impedances Z5 and Z6 should be selected to satisfy equation 12. These impedances may be of any type, but I prefer that capacitors be used. If desired, inductors and/or resistors may also be connected in series with each of the capacitors C5, C6, as long as equation 12 is satisifed.

While the circuit of FIG. 4 operates satisfactorily, it will be apparent that the primary winding of the transformer 30 is at a relatively high voltage. Accordingly suitable insulation between the primary and secondary windings should be provided for the transformer 30 to prevent this high voltage from breaking down the insu lation and appearing at the secondary winding. In those situations where it is preferred that the primary winding 6. of the transformer be at a relatively low potential. l have provided the circuit shown in FIG. 5. A comparison of FIG. 5 with FIG. 4 will show that the circuit of FIG. 5 has been reversed from top to bottom Thus, the measuring capacitor CM has been placed in the lower line and connected to the terminal T2, and the capacitors C5, C6 have been interchanged. The terminal T4 of the primary winding of the transformer 30 has been connected to the terminal T2, and the other terminal of the primary winding of the transformer 30 has been connected to the terminal TSas before. With these changes, the primary winding of the transformer 30 is at a relatively low potential, so that breakdown insulation requirements for the transformer 30 need not be so severe. Otherwise, the circuit of FIG. 5 operates in the same manner and with the same relationships as de-.

scribed in connection with the circuit of FIG. 4. With the terminal T2 connected to ground, the voltage at the terminal T5 relative to the terminal T2 can, if desired, be measured directly without the use of the transformer 30.

It will thus be seen that l have provided a new and improved circuit for giving a voltage measurement which is independent of frequency and which is independet of load magnitude. in addition, my circuit is relatively simple and requires only a capacitor, a load, and a measuring circuit. And, I have shown in FIGS. 3, 4, and 5 relatively simple circuits which provide such a measurement. if an absolute value of the line voltage V1 is desirable or necessary, the measured voltage may be multiplied by the ratio to give the absolute line voltage V1, taking into consideration of course any voltage transformation provided by the transformer 30. In summary, while my invention has been described with reference to a particular embodiment, it is to be understood that modifications may be made without departing from the spirit of my invention or from the scope of the claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a high voltage system having a line, a first terminal, a first capacitor of magnitude Cl connected between said line and said first terminal, a second terminal, a second capacitor of magnitude C2 connected between said first terminal and said second terminal, an improved arrangement for measuring the voltage between said line and said second terminal comprising:

a. a measuring capacitor of magnitude CM having a first end adapted to be connected to one of said terminals and having a second end;

b. first means connected to said first end of said measuring capacitor and adapted to be connected to the other of said terminals for indicating a first voltage between said other terminal and said first end of said measuring capacitor as a function of the ratio c. second means connected to said second end of said measuring capacitor and adapted to be connected to said other terminal for indicating a second volt- 7 age between said other terminal and said second end of said measuring capacitor as a function of the ratio d. and third means connected to said first and second means for indicating the difference between said first and second voltages. 2. In a high voltage system having a line, a first terminal, a first capacitor of magnitudeCl connected between said line and said first terminal, a second terminal, a second capacitor of magnitude C2 connected between said first terminal and said second terminal, an improved arrangement for measuring the voltage between said line and said second terminal comprising:

a. a measuring capacitor of magnitude CM having a first end adapted to be connected to one of said terminals and having a second end; b. a measuring terminal; c. a first impedance of magnitude Zl;

8 d. a second impedance of magnitude Z2; 0. said impedances and said capacitors having the following magnitude relation:

f. means connecting said first impedance between said second end of said measuring capacitor and said measuring terminal;

g. means connecting said second impedance between said measuring terminal and the other of said terminals;

h. and means connected between said one'terminal and said measuring terminal for deriving the, voltage therebetween. t v

3. The improved arrangement of claim 2 wherein said first and second impedances each comprise a capacitor.

4. The improved arrangement of claim 2 wherein said first and second impedances each consist of only a capacitor. 

1. In a high voltage system having a line, a first terminal, a first capacitor of magnitude C1 connected between said line and said first terminal, a second terminal, a second capacitor of magnitude C2 connected between said first terminal and said second terminal, an improved arrangement for measuring the voltage between said line and said second terminal comprising: a. a measuring capacitor of magnitude CM having a first end adapted to be connected to one of said terminals and having a second end; b. first means connected to said first end of said measuring capacitor and adapted to be connected to the other of said terminals for indicating a first voltage between said other terminal and said first end of said measuring capacitor as a function of the ratio
 2. In a high voltage system having a line, a first terminal, a first capacitor of magnitude C1 connected between said line and said first terminal, a second terminal, a second capacitor of magnitude C2 connected between said first terminal and said second terminal, an improved arrangement for measuring the voltage between said line and said second terminal comprising: a. a measuring capacitor of magnitude CM having a first end adapted to be connected to one of said terminals and having a second end; b. a measuring terminal; c. a first impedance of magnitude Z1; d. a second impedance of magnitude Z2; e. said impedances and said capacitors having the following magnitude relation:
 3. The improved arrangement of claim 2 wherein said first and second impedances each comprise a capacitor.
 4. The improved arrangement of claim 2 wherein said first and second impedances each consist of only a capacitor. 